Evaporative roof cooling is the science of cooling roof surfaces by the application of minute amounts of water to be evaporated, thus lowering the temperature of the structure. It is not a new idea, and in fact was practiced by Queen Semiramis of Babylon some 3000 years ago. Millions of square feet of roof have been cooled by modern systems since the first successful demonstration in 1934.
When properly engineered, such a system produces a water film of one one-thousandth of an inch on the treated roof. This is done by spraying a predetermined amount of water onto the surface. The time to apply the water is measured in seconds and the time for the water to evaporate is measured in minutes. The evaporation rate which will cause the heat resulting from average bright sunshine to be consumed is approximately 0.035 gal/hr sq ft of roof.
Past mechanisms for controlling the administration of water onto the roof have not provided precise 24-hour operation. They often fail to maintain the desired water film, resulting in a loss of cooling efficiency. Those mechanisms include simple timed or thermostatic controls.
Timers provide inadequate control since their operation does not vary with the temperature dependent evaporation rate.
Thermostatic controls, such as that disclosed in U.S. Pat. No. 2,506,936 issued to Murray, measure the temperature of the roof surface and actuate the wetting mechanism whenever that temperature rises above a predetermined point. They work moderately well in hot weather and bright sunshine, but do not provide truly precise 24-hour control. When the sun goes down and the dry bulb temperature no longer rises above that point, the system is rendered inoperative.